Small, inefficient modification to LiteDB's BsonMapper to allow recursive/mutually recursive database references.

BsonMapper.cs.diff

diff --git a/LiteDB/Mapper/BsonMapper.cs b/LiteDB/Mapper/BsonMapper.cs
index 91a7920e..c6dfee9a 100644
--- a/LiteDB/Mapper/BsonMapper.cs
+++ b/LiteDB/Mapper/BsonMapper.cs
@@ -389,11 +389,11 @@ namespace LiteDB
         /// </summary>
         private static void RegisterDbRefItem(BsonMapper mapper, MemberMapper member, string collection)
         {
-            // get entity
-            var entity = mapper.GetEntityMapper(member.DataType);
-
             member.Serialize = (obj, m) =>
             {
+                // get entity
+                var entity = mapper.GetEntityMapper(member.DataType);
+
                 // supports null values when "SerializeNullValues = true"
                 if (obj == null) return BsonValue.Null;

@@ -413,6 +413,9 @@ namespace LiteDB

             member.Deserialize = (bson, m) =>
             {
+                // get entity
+                var entity = mapper.GetEntityMapper(member.DataType);
+
                 var idRef = bson.AsDocument["$id"];

                 return m.Deserialize(entity.ForType,
@@ -427,11 +430,11 @@ namespace LiteDB
         /// </summary>
         private static void RegisterDbRefList(BsonMapper mapper, MemberMapper member, string collection)
         {
-            // get entity from list item type
-            var entity = mapper.GetEntityMapper(member.UnderlyingType);
-
             member.Serialize = (list, m) =>
             {
+                // get entity from list item type
+                var entity = mapper.GetEntityMapper(member.UnderlyingType);
+
                 // supports null values when "SerializeNullValues = true"
                 if (list == null) return BsonValue.Null;